Oxinium implants are alloys of zirconium and niobium with the surface oxidized by thermal diffusion to transform into a layer of zirconium oxide ceramic approximately 5 μm thick12. The transformed Oxinium surface layer has a reported advantage over CoCr in resisting roughening, frictional behavior, and biocompatibility6. It has also been reported to have lower wear than CoCr in TKA simulated wear studies6,23,24 and femoral component retrieval studies25,26.
In this age-stratified matched-cohort registry study, Oxinium femoral components did not reduce revision rates for all causes, loosening or lysis, or when infection as a cause of revision was removed, compared with the same CoCr femoral component, across all age groups, including patients who were <55 years old. There was also no difference between cohorts with respect to revision rates when patellar resurfacing or nonresurfacing was examined.
By focusing on the cemented Genesis-II cruciate-retaining TKA prosthesis (which has one of the lowest cumulative percent revision rates for a cemented cruciate-retaining knee prosthesis in the AOANJRR at 13 years3 and high survivorship rates in other studies27), the possible confounding prosthesis-related variables, such as femoral component design, fixation method, polyethylene, and constraint, have been removed.
Patient age is a well-recognized risk factor for revision TKA, with higher revision rates in younger patients3,20,28,29, not only because of aseptic mechanical failure, such as loosening or lysis, but also because of infection and pain. It has been suggested that Oxinium be utilized in these younger patients because of proposed improved wear characteristics10. In this matched cohort, patients who were <55 years old received almost double the proportion of Oxinium, possibly in the belief that Oxinium may provide younger patients with a reduced rate of revision. Stratification by age removed this as a possible confounder7, and young patients with Oxinium and CoCr prostheses had similar outcomes.
Measures to reduce the rate of revision of TKA implants because of wear-related issues are important to examine, particularly as the burden of revision falls disproportionally more on younger patients30. Improved survivorship of TKA components has been linked to a number of factors, including cross-linked polyethylene3,18, improved polyethylene sterilization31, surgeon volume3,29, surgeon training29, computer navigation32, patient age20, patellar resurfacing22,33, and prosthesis design27,29,34.
The clinical assessment of Oxinium as a potential means of reducing wear and associated clinical failure was justified by a number of preclinical studies. White et al.6 reported that Oxinium femoral components in a knee simulator had less polyethylene wear and no delamination in a 2-million-cycle test compared with the same CoCr total knee replacement. Ezzet et al.23 simulated polyethylene wear in athletically active patients with modestly malaligned Oxinium and CoCr TKA implants of the same design and reported that the mean gravimetric and volumetric wear rate after >5 million cycles was 55% lower in the oxidized zirconium group. The results of these simulator studies do not support the findings of the present study, and it remains uncertain why the improved wear characteristics of Oxinium do not result in a reduced revision rate. Essner et al.36 compared wear in a simulated stair-climbing load between the Genesis-II Oxinium TKA prosthesis and CoCr TKA components with a different femoral design. They reported a lower wear rate in the alternative prosthetic design and suggested that design geometry had the predominant effect on long-term wear rather than the materials chosen for the bearing. The results of that study support the hypothesis that prosthetic design is the first-order determinant of prosthesis survivorship, reinforcing the importance of arthroplasty registries.
The only difference between the cohorts was the revision rate due to loosening or lysis in patients ≥75 years old, which was lower in the CoCr cohort. It remains unexplained why these older patients had a higher revision rate because of loosening or lysis with an Oxinium femoral component when the cumulative percent revision for all causes and for aseptic causes were the same; however, it should be noted that our significance testing was not corrected for multiple comparisons.
This study has numerous strengths, including a large number of procedures and the use of >12 years of population-based data, making it, to our knowledge, the largest study with the longest follow-up to date to describe the outcomes after Oxinium TKA. Different rates of patellar resurfacing between the cohorts may have biased the data; however, by comparing revision rates in patients with and without patellar resurfacing, this potential confounder was removed. It is also important to note that we excluded cross-linked polyethylene from the analysis, as its follow-up was only short term. To include cross-linked polyethylene would have resulted in a heterogeneous group of bearing surfaces. Whether cross-linked polyethylene with Oxinium components would result in lower wear rates than cross-linked polyethylene with CoCr components remains uncertain.
There are limitations to our study. While this age-stratified, matched prospective cohort study contains a large number of patients, a limitation is its observational design. Unrecognized selection bias may have occurred because of surgeons choosing the Oxinium implant for patients at higher risk, such as those who were obese, highly active, or with self-described metal allergies. The Registry collects Type-I and II data37, and information on comorbidities and prosthetic alignment is currently not collected. Surgeons with less surgical experience or lower volume may have preferentially chosen the Oxinium implant, resulting in a higher proportion of component malalignment and a higher wear rate, despite the possibly improved wear characteristics. In addition, we cannot comment on the comparative revision rates for Oxinium compared with CoCr components at >12 years.
In conclusion, at 12 years, there was no advantage to the use of Oxinium compared with CoCr in TKA by overall revision rate, revision for loosening or lysis, or for revision for non-infection-related causes.
Investigation performed at the Australian Orthopaedic Association National Joint Replacement Registry
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